1. Field of the Invention
The present invention relates to a power source circuit and, more particularly, to a switching power source circuit suitable as a power source of a computer or the like.
2. Description of the Prior Art
A typical conventional switching power source circuit is illustrated in FIG. 1. Reference numeral T denotes a transformer. A primary winding L.sub.1 of the transformer T receives a voltage from a power source E switched by a switching transistor Q. A voltage is rectified by a secondary winding L.sub.2 appears at an output terminal OUT. At the beginning of operation, a current from the power source E flows in a capacitor C" and a feedback winding L.sub.3 through a resistor R.sub.10 to charge the capacitor C". The current from the power source E also flows in the base of the switching transistor Q. A current flows in the primary winding L.sub.1 and the collector of the switching transistor Q, thereby turning on this transistor. At the same time, a voltage is induced by the feedback winding L.sub.3 to supply a base current to the switching transistor through the capacitor C". In this case, the capacitor C" is charged with polarities opposite to those illustrated in FIG. 1. When the capacitor C" is gradually charged, the base current is decreased, so that the switching transistor Q is turned off. When the switching transistor Q is turned off, a counter voltage is generated by the feedback winding L.sub.3 to charge an electrolytic capacitor C through a diode D. When a charge voltage of the electrolytic capacitor C exceeds a predetermined voltage, a reverse bias voltage is supplied to the base of the switching transistor Q. When the switching transistor Q is turned off, a current supplied to the primary winding L.sub.1 is cut off. A voltage will not be generated from the feedback winding L.sub.3. As a result, the switching transistor Q is completely turned off.
When the capacitor C is discharged through a timing resistor R to release the transistor Q from the OFF state, a current flows in the base of the switching transistor Q through a starting resistor R.sub.10. The above operation is then repeated. In this manner, continuous oscillation is performed to extract a continuous output at the output terminal OUT. It should be noted that a Zener diode ZD is used as a protective element for the switching transistor Q.
In the conventional power source circuit, since oscillation is performed by utilizing a signal from the feedback winding L.sub.3, an oscillation frequency cannot be increased. The sizes of the transformer T and the electrolytic capacitor C cannot be reduced, and therefore the power source device is large and high in cost. In addition to these disadvantages, the efficiency of the device is degraded (in general, about 70%), and a load response time is long. Furthermore, when a voltage from the power source E is decreased or the output terminal OUT is overloaded, a voltage at the output terminal OUT is lower than the predetermined voltage. A load (e.g., a computer) is adversely affected. An oscillation period of the switching transistor Q is determined by a time constant of an entire feedback circuit including an inductance of the feedback winding L.sub.3. Therefore, the oscillation period cannot be finely adjusted, resulting in inconvenience.